SEATTLE (AP) — Federal authorities aren’t sure yet who shot several sea lions in the Puget Sound region in recent weeks, but a conservation group has offered up $10,000 for information that helps convict the perpetrators.

At least seven dead sea lions and one harbor seal have washed up in Puget Sound and on the Washington coast since early December, Brian Gorman, a spokesman with NOAA Fisheries Service in Seattle, said Tuesday. One of the animals was a federally endangered Stellar sea lion.

Bullets or bullet fragments have been found or recovered from all seven sea lions, but biologists are working to determine whether the bullets killed them, Gorman said. He noted that stranded mammals have been found with old bullet wounds that aren’t fatal.

The number of dead marine mammals is unusual but not alarming so, authorities say.

“This is not uncommon to have dead marine mammals killed intentionally,” said Mike Cenci, deputy enforcement officer with the Washington Department of Fish and Wildlife, which is assisting NOAA in the investigation. “I’m not saying it’s rampant, but we investigate a number of these a year.”

Salmon-hungry sea lions often get entangled in fisherman’s nets or catches, and some consider them to be nuisances.

“They’re not warm and cuddly. These are big guys. They’re stinky. They’re carnivores, and they’re doing what they do best — they’re hunting for fish,” which puts them in conflict with fishermen, Gorman said.

In biological terms, however, losing this many animals won’t have a measureable effect on the population of California sea lions, which is very healthy, he said.

All the animals are federally protected by the Marine Mammal Protection Act. But Steller sea lions are also protected under the Endangered Species Act.

A necropsy is being performed Tuesday on one sea lion, and X-rays are scheduled on three others this week. Bullets recovered from the animals could also help determine whether the bullets came from the same gun, he said.

Investigating such marine mammal deaths can be challenging, because very often there are no witnesses and such crimes often happen on the water where few are watching, officials said.

Bill Giles, deputy special agent in charge of Northwest enforcement for NOAA, said officers are following tips received through its hotline.

Many of the animals were dead before they were found. One dead lion discovered last week in West Seattle had been dead for at least three weeks.

The Sea Shepherd Society is urging people with information to call NOAA’s national enforcement at 800-853-1964 or call 911 to report a crime in progress.

In 2008 when six sea lions were found dead in traps on the Columbia River, suspicion immediately fell on fishermen, who have long complained that sea lions gobble up salmon at the Bonneville Dam. Federal officials initially said the sea lions had been shot, but necropsies by state and federal experts later found no evidence of bullet wounds. NOAA later said the deaths of those animals were the results of necropsies on all six animals were consistent with death from heat stroke.